Microsoft Windows: Heap-based buffer overflow in Client License Service
Summary
A heap-based buffer overflow in the Windows Client License Service allows an authorized local user with low privileges to elevate privileges to a higher level on the system. Exploitation requires local access and a valid low-privileged account, so the flaw is not directly reachable from the internet. The CNA and NVD rate it as High (CVSS 7.0), with high impact on confidentiality, integrity and availability.
Affected
- Windows 10 Version 1809 (10.0.17763.0 to below 10.0.17763.9376)
- Windows Server 2019 and Server Core installation (10.0.17763.0 to below 10.0.17763.9376)
- Other affected Windows 10, Windows 11 and Windows Server versions are listed in the MSRC advisory
Fixed in
- Windows 10 Version 1809 and Windows Server 2019: build 10.0.17763.9376 or later, included in the October 2026 security update (KB5126651)
- Build numbers for other versions: see the MSRC advisory
Workaround
Unknown. No vendor mitigation is documented in the sources reviewed. Installing the October 2026 cumulative update is the documented remedy.
Exploitation
- No evidence of exploitation in the wild was found for this CVE in the sources reviewed.
- CISA SSVC: Exploitation none, Automatable no, Technical Impact total.
- No public proof of concept found.
- EPSS score 0.0027.
Our recommendation
- Install the October 2026 cumulative update (KB5126651 for Windows 10 1809 and Server 2019) and verify the build is 10.0.17763.9376 or later.
- Identify endpoints on shared PCs, terminal or RDS servers and multi-user systems first, as these are most at risk.
- Check whether devices are on legacy or unsupported editions and plan an upgrade.
- Re-check the MSRC advisory for changes to exploitation status.
Information from Microsoft (Patch Tuesday October 2026)
- Severity (Microsoft)
- Important
- Impact
- Elevation of privilege – Windows Client License Service
- Status on Patch Tuesday
- Neither exploited nor publicly disclosed
- Exploitability assessment
- Exploitation less likely
- Product family
- Windows, Windows ESU (Extended Security Updates)
- Updates (KB)
- KB5126651, KB5126657, KB5126655, KB5126653, KB5127648, KB5126646
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Microsoft Security Update Guide: CVE-2026-100201 msrc.microsoft.com
- NVD record via Vulners vulners.com
- CVE List (CNA record) via Vulners vulners.com
- Tenable CVE-2026-100201 plugins www.tenable.com
- Strix CVE-2026-100201 www.strix.ai
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 4 Nov 2026, 10:03.
Original description (NVD)
Heap-based buffer overflow in Windows Client License Service allows an authorized attacker to elevate privileges locally.
NVD references (1)
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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